David Mann is a middle-aged salesman driving on a business trip. He encounters a dilapidated tanker truck in the Mojave Desert. Mann passes the truck but the truck speeds up and roars past him. When Mann overtakes and passes it again, the truck blasts its horn. Mann leaves it in the distance.

Mann pulls into a gas station and shortly afterward the truck arrives and parks next to him. Mann phones his wife, who is upset with him after an argument the previous night. The station attendant tells Mann he needs a new radiator hose but Mann doesn’t believe the attendant’s sincerity and declines the repair.

Back on the road, the truck catches up, passes then blocks Mann’s path each time he attempts to pass. After antagonizing Mann for a while, the driver waves him past but Mann nearly hits an oncoming vehicle. Mann finally passes the truck using an unpaved turnout next to the highway then glances at his rear window and waves as the speed of the truck decreases.

The truck then tailgates Mann’s car at increasingly high speed. Mann swerves his car off the road, loses control, and crashes sideways into a fence across from a diner as the truck continues down the road. Mann then enters the restaurant to compose himself. Upon returning from the restroom, he sees the truck parked outside. He studies the patrons and confronts one he believes to be the truck driver. The offended patron beats Mann and leaves in a different truck. The pursuing truck leaves the diner seconds later, indicating that its driver had never entered the premises.

Mann leaves the diner and soon stops to help a stranded school bus but his front bumper gets caught underneath the bus’s rear bumper. The truck appears at the end of a tunnel, causing Mann to panic. He and the bus driver then free his car and Mann drives from the scene as the truck helps push the school bus onto the road. Shortly after, down the road, Mann stops at a railroad crossing waiting for a freight train to pass through. The truck appears from behind and pushes Mann’s car towards the oncoming Southern Pacific freight train. The train passes, and Mann crosses the tracks and pulls over. The truck continues down the road and Mann slowly follows.

In an attempt to create more distance between him and the truck, Mann drives at a very leisurely pace, as other motorists pass him. Once again, he encounters the truck, which has pulled off to the side of the road ahead, intentionally waiting for Mann. He pulls out in front of him and starts antagonizing him again.

Mann stops at a gas station/roadside animal attraction to call the police and replace his radiator hose but when he steps into the phone booth, the truck drives into it, but Mann jumps clear just in time. The station owner cries out as the truck destroys her animals’ cages. Mann jumps into his car and speeds away. Around a corner, he pulls off the road, hiding behind an embankment as the truck drives past.

After a long wait, Mann heads off again but the truck is waiting for him again down the road. Mann attempts to speed past but it moves across the road, blocking him. Mann seeks help from an elderly couple in a car but they flee when the truck backs up towards them at high speed. The truck stops before hitting Mann’s car and Mann speeds past the truck, which begins pursuing. Mann swerves towards what he believes is a police car, only to see it is a pest-control vehicle. The truck chases him up a mountain range. The faulty radiator hose of Mann’s car breaks, causing the strained engine to overheat and begin failing. Losing speed, he barely reaches the summit but then coasts downhill in neutral as the truck follows.

Mann spins out and crashes into a cliff wall, barely escaping being crushed by the truck. He manages to restart his car, then drive up a dirt road with the truck following him. He turns to face the truck in front of a canyon, locks the accelerator using his briefcase, then steers the car into the oncoming truck, jumping free at the last moment. The truck hits the car which bursts into flames, obscuring the driver’s view. The truck plunges over the cliff, along with the car, as the driver sounds the truck’s horn. Above the wreckage, Mann celebrates. He then sits at the cliff’s edge and throws stones into the canyon as the sun begins to set.